Stabilizing control and controllability:

1. Stabilizing control and controllability: Control solutions to avoid slug flow in pipeline-riser systems Espen Storkaas Trondheim 7.6.2005

2. Thesis summary • Introduction • Controllability analysis of a two-phase pipeline-riser systems at riser slugging conditions • A low-dimensional dynamic model of severe slugging for control design and analysis • Implications of input rate limitations on controllability and controller design • Stabilization of multiphase flow in pipelines with single-loop and cascade controllers • Model-based anti-slug controllers • Extended slug control – An industrial application • Conclusions and further work

3. Outline • Introduction • Slug flow in pipeline-riser systems • Modelling of pipeline-riser systems for control applications • Controllability analysis • Effect of input rate limitations • Controller design • Extended slug control • Conclusions

4. Introduction • Oil producing wells also produce gas and water • Longer multiphase tie-in lines in offshore oil productionfrom increases flow-related challenges • Flow assurance technology plays an increasingly important role • Hydrates • Wax • Corrosion • Flow regimes

5. Slug flow in pipeline-riser systems • Riser slugging • Hydrodynamic slugging ....... • Terrain slugging • Transient slugging *Pictures from SINTEF Multiphase flow laboratory

6. Riser slugging and control - History • From design challenge to control objective • First relevant publication : Schmidt et al (1979) • Experimental work by Hedne & Linga (1990) • Simulations studies and experimental work from several sources (Total, Shell, ABB, Statoil) • First industrial application: Hod-Valhall (Havre et al 2000), more has followed in later years • Included in design of new projects (riser slugging potensial at design conditions)

7. Outline • Introduction • Slug flow in pipeline-riser systems • Modelling of pipeline-riser systems for control applications • Controllability analysis • Effect of input rate limitations • Controller design • Extended slug control • Conclusions

8. Main case study • Test case for riser slugging in OLGA • Simplified geometry • Two-phase flow • Constant feed • Constant pressure behind choke

9. Bifuracation diagram for riser slugging

10. Modelling (1) – Lessons learned from two-fluid model • Two-fluid model used to investigate system caracteristics • Transition to instability through Hopf bifurbation • Complex unstable poles • Controllability analysis gives information about measurement selection • Simpler model should be used

11. Modelling (2)- Design specs for simplified model • The model must: • Describe the dominant dynamic behavior of the system for the time scales for which control is to be effective • The model should : • be continuous • be simple (low state dimension) • contain few empirical coefficients

12. Modelling (3) – Simplified 3-state model Three dynamical states From entrainment model Given by valve equation:

13. Modelling (4) – Entrainment model

14. Modelling (5) –Properties of 3-state model • Phenomenological model with 3 dynamical states • Based on bulk properties • Describes both riser slugging and unstable stationary operating points • 4 empirical parameters – easy to tune • Hopf bifurcation, complex unstable poles • Very useful for controllability analysis and controller design

15. Modelling (6) – Model comparison

16. Outline • Introduction • Slug flow in pipeline-riser systems • Modelling of pipeline-riser systems for control applications • Controllability analysis • Effect of input rate limitations • Controller design • Extended slug control • Conclusions

17. Inverse response Controllability analysis • Investigation into a plants achievable control performance • Independent of controller • Step in valve opening:

18. Measurement evaluation • Achievable performace can be represented by lower bounds on closed-loop transfer functions such as • Sensitivity function S • Complementary sensitivity function T • Input usage KS • Bound computed from 3-state model • Small numerical value for lower bounds on closed loop transfer functions indicate a good measurement candidate

19. Measurement evaluation Unstable system at 30% valve opening pi=0.0007±0.0073 Low steady-state gain Similar results from two-fluid model

20. Conclusions from controllability analysis • Inlet or riserbase pressure well suited for stabilizing control • Time delay may prevent the use of inlet pressure for long pipelines • Pressure at top of riser not suitable for stabilizing control due to unstable zero dynamics • Flow measurement at riser outlet can be used for stabilization but has lacking low-frequency gain • Best used as a secondary measurement in a cascade or in combination with another measurement

21. Outline • Introduction • Slug flow in pipeline-riser systems • Modelling of pipeline-riser systems for control applications • Controllability analysis • Effect of input rate limitations • Controller design • Extended slug control • Conclusions

22. Effect of input rate limitations • Limitation on input rate can limit performance for control systems • Explicit lower bounds on required input rate derived • Stabilization • Disturbance rejection • Controller design with limited input rates

23. Controller design • Controllers design based on simplified 3-state model • Stabilizes both two-fluid model and OLGA model • Measurement selection from controllability analysis confirmed • Controllers based on upstream pressure measurement robust and effective • Controllers based on only a flow measurement tends to drift off • A flow measurement combined with another measurement can be used for stabilizing control

24. Single-loop controllers PID controller with measured inlet pressure H∞controller with measuredinlet pressure PID controller with measuredriser base pressure PID controller with measuredoutlet flow

25. Cascade and MISO controllers Cascade controller, y1=PI, y2=Q H∞-controller, y=[DP Q] Cascade controller, y1=DP, y2=Q

26. Extended slug control • Anti-slug control combined with functionality to mitigate surge waves and startup slugs

27. Summary • Simplified model of pipeline-riser systems at riser slugging condisons for controllability analysis and controller design • Controllability analysis gives clear recommendations for measurement selection for stabilizing control • Input rate limitations may be important • Controller design • Extended control application Further work • Effect of water, different geometries • New measurements